Structures and devices designed to work in the frequency range of 0.1-10 THz are proved to be essential in imaging, spectroscopy, biosensing and other applications alike. They are also strong candidates for building the next-generation integrated circuits that will eventually close the gap between electronics and optics. Concentration of terahertz radiation in small volumes can facilitate the sub-wavelength transmission of the signal, generating new possibilities for low-loss, low-dispersion delivery of terahertz radiation over distance. The strong confinement of the E-M field can also enhance the signal-matter interaction, hence maximizing the modulation efficiency in active device designs.
Strong mode localization at terahertz frequency is realized by adding periodic surface features onto the material interface of conventional waveguide structures. Unlike using resonant structures with dimensions comparable with the wavelength, the metamaterial created by the sub-wavelength surface modifications can support a special surface mode, named spoofed surface plasmon polariton (SSPP) mode. With discrete transmission peaks and valleys, the SSPP modes can be modulated by changing geometric dimensions and material parameters of the structure. In most of the studies employing SSPP terahertz architectures, the discrete passing bands and their amplitude modulation due to external stimuli serve at the backbone of the device functionality.
Kramers-Kronig relationship dictates coupled evolution of the real and imaginary parts of the dielectric constant. Therefore, the change in transmittance must be accompanied with the shift in phase accumulation. The inclusion of phase information is, hence, critical in the effort to achieve higher resolution and sensitivity for the SSPP terahertz devices. A metamaterial terahertz phase modulator based on metallic split-ring resonator design has been proposed. In the proposed design, voltage across the Schottky contact formed by the doped semiconductor (GaAs) layer and metal electrode causes depletion of free carriers, changing the complex transmittance of the structure at terahertz frequencies. Phase modulators in the form of waveguides have also been proposed and fabricated by a number of research groups, where doped Si is used to form a thin strip ring resonator. The phase of the propagating signal can be adjusted by free carrier injection through a p-i-n junction, and as a result, the coupling efficiency of the ring resonator can be modulated. Due to the large loss tangent of Si in terahertz domain however, such design can only be effectively applied up to near-infrared frequencies.
In this disclosure, a terahertz beam bender is presented that can be actively controlled through free carrier density modulations. Based on this phenomenon, a multibit analog-to-digital converter (ADC) can be realized that utilizes terahertz signals. The disclosure is organized as follows. First, the mathematical analysis of the DC-SSPP structure will be briefly described and phase modulation with refractive index change in a simplified model will be presented. Next, finite-element simulations for the terahertz beam bender operating in the enhancement mode and depletion mode will be discussed, followed by a section on design and analysis of the ADC.
This section provides background information related to the present disclosure which is not necessarily prior art.